This invention relates generally to methods and arrangements for reducing partial discharges on printed circuit boards, and more particularly, to methods and arrangements for reducing partial discharges on printed circuit boards used in high voltage generators.
In high voltage generators used in medical imaging systems, such as, for example, an X-ray generator, high voltage DC is normally generated using a multiplier/doubler circuit operating at high frequency. The output voltage of the transformer is typically in tens of kilovolts (kV) and the operating frequency is in several tens of kilohertz (kHz). The rectifier units may include cascaded configurations to achieve a higher output voltage, as a higher output power is often needed for high quality X-ray generation. The operation of the rectifier units at these very high voltage levels and frequencies may result in high electrical and thermal stresses around the components on printed circuit boards (PCB).
Multiplier circuits are known to include components such as transformer coils, diodes, and capacitors mounted on a PCB forming a rectifier assembly and encased for example, within a polypropylene casing filled with oil. The oil around the multiplier PCB inside the polypropylene casing acts as a coolant and insulation.
Components and PCBs used in high and low voltage applications are likely to operate at very high stress levels. A configuration to reduce these stress zones requires adequate clearance and creepage distance between components mounted on the PCB. The amount of clearance depends on the breakdown strength of the surrounding medium such as air or oil. The creepage distance depends on the electric stress at the PCB surface and its interface with the ambient medium.
Further, and for example, in a multiplier PCB, the connections, for example, the transformer secondary coil connection to the diode through solder on the PCB (triple junction formed with solder, PCB and oil) and other diode solder points form high stress zones having electric stress. These high stress zones may cause partial discharges (PD) on the surface of the PCB that may be further enhanced at high temperature. Partial discharge and high temperature stresses together deteriorate transformer oil and PCB surface. Partial discharges may cause flashover between various components on the PCB if creepage distance is not adequate or cause puncture in the PCB if there is a significant accumulation of charges due to partial discharge on the surface of the PCB.
Thus, known methods of component mounting and connection may not provide compact assembly of components in a PCB in high power applications. Further, these may not provide reduced partial discharge conditions on the PCB.